First is the initialization of the button to be used in playing the sound. We use the Activity's findViewById, passing the Id we assigned to it (in this example's case: R.id.button1), to get the button that we need. We cast it as a Button so that it is easy to assign it to the variable one that we are initializing. Explaining more of how this works is out of scope for this answer. This gives a brief insight on how it works.

final MediaPlayer mp = MediaPlayer.create(this, R.raw.soho);

This is how to initialize a MediaPlayer. The MediaPlayer follows the Singleton Design Pattern. To get an instance, we call its create() method and pass it the context and the resource Id of the sound we want to play, in this case R.raw.soho. We declare it as final. Jon Skeet provided a great explanation on why we do so here.

Finally, we set what our previously initialized button will do. Play a sound on button click! To do this, we set the OnClickListener of our button one. Inside is only one method, onClick() which contains what instructions the button should do on click.

public void onClick(View v) {
mp.start();
}

To play the sound, we call MediaPlayer's start() method. This method starts the playback of the sound.

There, you can now play a sound on button click in Android!

Original Post

First. You should put your statements inside a block, and in this case the onCreate method.

Second. You initialized the button as variable one, then you used a variable zero and set its onClickListener to an incomplete onClickListener. Use the variable one for the setOnClickListener.